1. Field of the Invention
The invention relates to audio signal monitoring systems and, more particularly, to a method and indicating instrument for providing an operator with an coordinated display of two audio signal characterisitcs indicative of instantaneous signal level and coordinated signal energy.
2. Description of the Prior Art
In the fields of broadcasting and recording, an instrument capable of indicating the amplitude of the sound signal has been necessary to permit control of the dynamic range of the total audio system -- constrained at one extreme by the inherent noise level and at the other extreme by the onset of unacceptable distortion of the wanted signal. The problem is intensified by the fact that the human ear has a logarithmic intensity characteristic. Also, the ear judges loudness of sound largely by comparison with immediately preceding sound levels. The ear's subjective impression of loudness is largely determined by the total energy in the sound wave and the instantaneous peak sound pressure. Thus not only amplitude but also the wave shape and symmetry of the sound signal influences the ear and consequently judgments of intensity. Human speech and percussive sounds, for example, contain many short duration, high level peaks, while music generally, and especially the sounds of stringed and wind instruments, have less "spiky" or more symmetrical waveforms. Nevertheless, these two types of vastly differing waveshapes may have the same apparent loudness.
The problem of controlling broadcasting and recording volume thus is a matter of compromise. It is necessary to compress the intensity range in such a way that the original apparent loudness values are preserved as much as possible, yet at the same time make use of the fullest possible modulation depth of the broadcasting or recording medium.
The VU (volume unit) meter previously used for sound measurements in connection with volume control is patterned on so-called standards defined for it and agreed on by the industry. Thus its rectifier characteristics are in the form I=KE where E equals the applied voltage, I the current passed, and K a constant. The meter is scaled so that the 100% mark is at approximately 0.74 of the full scale deflection. The impedance of the meter and rectifier when indicating 100% under the influence of an applied sinusoidal voltage is 3,900 ohms. The sensitivity of the combination shall be such that when 1.228 volts r.m.s. is applied in series with an additional resistance of 3,600 ohms, the indication is typically between the 100% mark and 0.8 of full scale. The speed of the response is such that if 1.228 volts r.m.s. is suddenly applied, the pointer attains 99% of its final deflection within 0.3 seconds with an overswing of not less than 1% or more than 1.5%. With the overall impedance of the meter circuit being 7,500 ohms, the meter indicates 100% when a tone level of +4 dbm (decibels, referred to 1 milliwatt) is present across the standard 600 ohm feed line.
The VU meter, when bridged across the 600 ohm feed line, has been found to load the line and to cause frequency and level sensitive disturbances on the line which can be significant in a complex system using many VU meter circuits. Being an averaging or integrating device, the VU meter embodies ballistics which will not allow it to provide accurate indication of absolute levels of signals less than approximately 80 milliseconds in duration. The indication further appears to depend on waveform since the asymmetrical character of waveforms produced by the human voice in speaking and singing cannot be indicated as accurately as the more symmetrical music waveforms due to averaging characteristics of the meter ballistics. Also, the rectifiers in VU meters follow a square law type curve and thus are non-linear. These undesirable characteristics of the standard VU meters do not provide the information which recording mixers and operators require to attain the most satisfactory results, especially in view of the demands of today's unusual sounds in program material.